Sr2B5O9OH{middle dot}H2O, A SYNTHETIC BORATE RELATED TO HILGARDITE

The crystal structure of a new strontium borate, Sr 2 B 5 O 9 OH·H 2 O, space group C 2, a 10.2571(6), b 8.0487(2), c 6.4043(4) A, β 127.860(2)°, Z = 2, has been determined by X-ray diffraction using a single crystal grown under hydrothermal conditions. The structure is closely related to that of the zeolite-like hilgardite minerals, Ca 2 B 5 O 9 Cl·H 2 O, in particular that of triclinic hilgardite-1 A . The main structural differences are a more symmetrical conformation of the [B 5 O 12 ] 9− pentaborate chains and a random distribution of OH − and H 2 O in Sr 2 B 5 O 9 OH·H 2 O, that result in the monoclinic symmetry. The role of the Sr cations in determining the conformation of the pentaborate chain is discussed by comparison with hilgardite-1 A .